The present invention relates to shape memory actuators, i.e. actuators in which the actuating member consists of an element (for example a wire element) made from a shape memory alloy (indicated in the following as “SMA”), and in particular to an actuator in which the driven element is multistable, preferably bistable, i.e. it is moved by a driving element between at least two stable positions. Although specific reference is made in the following to the use of a wire as actuating member, it should be noted that what is being said also applies to other similar elongated shapes, i.e. with a dimension much greater than the other two dimensions which are generally very small, e.g. strips and the like.
It is known that the shape memory phenomenon consists in the fact that a mechanical piece made of an alloy that exhibits said phenomenon is capable of transitioning, upon a temperature change, between two shapes that are preset at the time of manufacturing, in a very short time and without intermediate equilibrium positions. A first mode in which the phenomenon may occur is called “one-way” in that the mechanical piece can change shape in a single direction upon the temperature change, e.g. passing from shape A to shape B, whereas the reverse transition from shape B to shape A requires the application of a mechanical force.
On the contrary, in the so-called “two-way” mode both transitions can be caused by temperature changes, this being the case of the application of the present invention. This occurs thanks to the transformation of the micro-crystalline structure of the piece that passes from a type called martensitic (M), stable at lower temperatures, to a type called austenitic (A), stable at higher temperatures, and vice versa (M/A and A/M transition).
A SMA wire has to be trained so that it can exhibit its features of shape memory element, and the training process of a SMA wire usually allows to induce in a highly repeatable manner a martensite/austenite (M/A) phase transition when the wire is heated and to induce an austenite/martensite (A/M) phase transition when the wire is cooled. In the M/A transition the wire undergoes a shortening by 3-5% which is recovered when the wire cools down and through the A/M transition returns to its original length.
This characteristic of SMA wires to contract upon heating and then to re-extend upon cooling has been exploited since a long time to obtain actuators that are very reliable and silent. In particular, this type of actuator is used in some valves to perform the movement of the shutter from a first stable position of closed valve to a second stable position of open valve, or to multiple stable positions of partially open valve, and vice versa.
Examples of valves with SMA actuators can be found in U.S. Pat. No. 6,840,257, U.S. Pat. No. 6,843,465, U.S. Pat. No. 7,055,793, U.S. 2005/0005980 and U.S. 2012/0151913. All these prior art documents disclose actuators that are quite complicated, bulky and rather expensive, usually involving the use of two SMA wires and/or mechanical stabilization elements such as a diaphragm for moving the shutter between the two (or more) stable positions. These types of known SMA actuators are therefore unsuitable to be scaled down in size and not fully reliable when used in harsh environments due their rather delicate and sophisticated operation.
SMA actuators are used also in a variety of other devices in which their operation is quite different from the two-way operation mentioned above.
U.S. 2007/0028964 discloses a resettable bi-stable thermal control valve that closes when fluid conducted therethrough reaches a predetermined temperature, so as to act as over temperature shut-off valve. More specifically, the reaching of the threshold temperature causes a SMA wire to contract and exert a force on an inner piston body to move into a piston cap compressing an inner piston spring, until two apertures provided through sidewalls of the piston cap become aligned with cavities formed in the piston body thus allowing corresponding balls to move from the outer surface of the piston body into said cavities, which in turn permits a shutter-carrying member that was previously blocked by said balls to retract into the valve body under the force of a spring.
This operation of the SMA wire causes an irreversible closure of the valve since the balls in the cavities are prevented by the shutter-carrying member from recovering their original position under the action of the inner piston spring even after deactivation of the SMA wire. This valve is therefore merely a safety device in which the SMA actuator is used only as a release mechanism, and such a device must be reset manually by pulling out the shutter-carrying member against the resistance of its spring until the apertures in the piston cap are cleared, such that the balls can recover their original position when the inner piston body moves out of the inner piston cap under the force of the compressed inner piston spring.
A similar use of a SMA actuator as a release mechanism is also disclosed in U.S. 2012/0187143 wherein a SMA wire is used to disengage the latch of a spring-loaded lid which is then re-closed manually. In this case the tension on the SMA wire is provided by two spring-loaded rotating levers that engage the wire through a capstan and a plunger.
Still another use of a SMA actuator as a release mechanism is disclosed in U.S. 2008/002674 wherein a SMA wire is used to disengage the latch of a door or trunk and a mechanism is provided to make use of the user's force in closing the door or trunk to restore the martensitic state of the SMA wire through a stress-induced state change in case the ambient temperature is so high that the SMA wire does not cool to the martensite transition temperature upon deactivation.